Piston arrangement

ABSTRACT

A piston arrangement is provided. The piston assembly includes a track and a piston moveable within a cylinder. The track is rotatable relative to the cylinder about an axis of rotation and has a cam surface and an edge surface extending away from the cam surface. The piston is coupled to the track by a follower running on the cam surface. The cam surface is shaped such that, as the track moves relative to the cylinder, the piston head moves in reciprocating motion within the cylinder along a piston axis in accordance with the path of the cam surface. The piston axis is perpendicular to the track axis of rotation. A stabilizing element is connected to the piston, the stabilizing element extending below the piston head and comprising a contact surface which engages the edge surface of the track.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of PCT/GB2015/050050,filed Jan. 13, 2015, which claims priority to Great Britain PatentApplication No. 1400682.9, filed Jan. 15, 2014, each of which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates to piston arrangements, in particularpower transfer mechanisms coupling a piston head to a track whichcontrols movement of the piston head.

BACKGROUND OF THE INVENTION

A conventional piston arrangement used, for example, in internalcombustion engines comprises a piston head moveable within a cylindercoupled to a crankshaft by a con-rod. In an alternative pistonarrangement a piston head may be coupled to a track having a camsurface, the piston head being provided with one or more cam followerswhich run along the track to control movement of the piston head. Thepresent invention relates to improvements in the power transfermechanism coupling a piston head to a track in such an alternativepiston arrangement.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a piston arrangement includinga track and a piston moveable within a cylinder; wherein the track isadapted to rotate relative to the cylinder about an axis of rotation andhas a cam surface and an edge surface extending away from the camsurface; wherein the piston is coupled to the track by a followerrunning on the cam surface; wherein the cam surface is shaped such that,as the track moves relative to the cylinder, the piston head moves inreciprocating motion within the cylinder in accordance with the path ofthe cam surface; wherein a stabilising element is connected to thepiston, the stabilising element extending below the piston head andcomprising a contact surface which engages the edge surface of thetrack. The stabilising element acts to stabilise the piston a directionperpendicular to the plane of the edge surface of the track.

The track may rotate about the axis of rotation while the cylinderremains stationary with respect to the axis of rotation. Alternativelythe cylinder may move along a fixed track. The track may comprise asingle track portion or alternatively at least two track portions onopposite sides of the stabilising portion. The or each track portion mayprovide a cam surface on which a single follower runs or alternativelytwo opposing cam surfaces on which first and second followers run.Various different possible track arrangements will be discussed ingreater detail below.

The track may form a continuous loop and the cam surface may form acontinuous surface extending around the loop. Alternatively there may bea break in the cam surface, for example a small discontinuity. The camsurface may be fixed with respect to the track, or alternatively the camsurface may include a fixed portion which is fixed with respect to thetrack and a moving portion which forms part of the cam surface and whichis moveable with respect to the track, for example one or more rollersmounted to the track. The roller may be adapted to rotate about an axisof rotation, and may rotate freely, be partially constrained and/or bedriven.

The stabilising element may be rigidly attached to the piston head, forexample by fasteners. Alternatively the stabilising element may beotherwise rigidly or pivotally attached to the piston head or may beintegrally formed with the piston head. The stabilising element may beconnected to the piston head via an intermediate component such as acon-rod, which may be rigidly attached to the piston head and to thestabilising element or alternatively pivotally attached to the pistonhead and to the stabilising element.

The stabilising portion may be blade-like such that it has a thicknessin a direction perpendicular to the plane of the edge surface of thetrack which is significantly smaller than its length in a directionparallel to the plane of the edge surface and perpendicular to thedirection of movement of the piston. For example, the length of thestabilising element may be at least twice the thickness of thestabilising element, or at least three times the thickness, or at leastfour times the thickness, or at least five times the thickness. Thelength of the stabilising element may be substantially equal to thediameter of the piston head, or alternatively may be greater or smallerthen the piston head diameter.

The shape of the contact surface of the stabilising element maysubstantially correspond to the shape of the edge surface of the track.For example the edge surface of the track may be straight and thecontact surface of the stabilising element may have a correspondingstraight shape, or alternatively the edge surface of the track may beconvex and the contact surface of the stabilising element may have acorresponding concave shape. Alternatively the contact surface of thestabilising element may have some other shape, for example a convexstabilising element may bear against a straight or convex edge surfaceof the track.

The stabilising element may comprise first and second end surfaces, thecontact surface extending between the first and second end surfaces,wherein at least one of the first and second end surfaces slidinglyengages a locating element as the piston moves in reciprocating motion.Preferably the first and second end surfaces of the stabilising elementslidingly engage first and second locating elements, thereby stabilisingthe piston in a direction perpendicular to the plane of the edgesurface(s) of the track. The interface between the end surfaces and thelocating elements is preferably planar but may have some other shape.

The follower may comprise a roller mounted to the piston, the rollerbeing adapted to rotate relative to the piston as it runs along thetrack. The roller may be mounted in or on the piston head and/or in oron the stabilising element. The piston arrangement may, for example,comprise a bearing according to the second aspect of the invention(discussed below). Alternatively the follower may be a be a rollermounted on a full cylindrical shaft which protrudes from the piston headand/or stabilising element. Alternatively the follower may be a rollerextending through or into the piston head and/or stabilising elementwhich only engages the piston head and/or stabilising element via a fullcylindrical contact patch extending around the circumference of theroller. There may be multiple rollers or other followers mounted to thepiston, for example multiple followers running on multiple cam surfacesof the track.

The track may comprise first and second cam surfaces facing away fromeach other in opposite directions, the edge surface extending betweenthe first and second cam surfaces; wherein the piston comprises firstand second followers respectively running on the first and second camsurfaces. The first and second followers are preferably offset from eachother is a direction parallel to the piston axis. The stabilisingelement preferably extends between and connects the first and secondfollowers. The first follower may be arranged to move the piston in afirst direction aligned with its axis and the second follower may bearranged to move the piston in a second direction opposing the firstdirection. The opposing first and second cam surfaces may oppose eachother by facing outwardly away from each other or alternatively byfacing inwardly towards each other.

The track may comprise first and second track portions located onopposite sides of the stabilising element, wherein the first trackportion comprises an edge surface extending away from at least one camsurface and the second track portion comprises an edge surface extendingaway from at least one cam surface and facing towards the edge surfaceof the first track portion, wherein the stabilising element comprisesfirst and second contact surfaces which engage the respective edgesurfaces of the first and second track portions. The piston maytherefore be fully stabilised in a direction perpendicular to the planeof the edge surfaces of the track.

The stabilising portion may have a substantially uniform thickness alongits length. The stabilising element may have a thickness which tapersacross the extent of the stabilising element in a direction parallel tothe direction of relative movement between the piston and the track. Thetaper may allow the stabilising portion to be self centering within agap formed between the edge surfaces of the first and second trackportions, for example under hydro-static pressure. The taper may besmall compared to the overall thickness of the stabilising element, forexample a clearance of 0.08 mm may be provided between the contactsurfaces of the stabilising element and the edge surfaces of therespective track portions at one end of the stabilising portion comparedto a clearance of 0.04 mm at the opposing end. Different clearances andtaper angles may be employed depending on the application of the pistonarrangement. The stabilising element may alternatively have any othershape, for example a convex shape bearing against a straight or convexedge surface.

The track may be a radial track with the cam surface(s) arranged asinner radial and/or outer radial surfaces with respect to the axis ofrotation. The track may therefore have a height in a direction extendingradially outwardly from the axis of rotation which varies along theextent of the track. The edge surface of the first track portion may besubstantially parallel to the edge surface of the second track portion.The stabilising element may be substantially straight when viewed from adirection parallel to the axis of the piston.

The track may be an annular track with the cam surface(s) arrangedfacing in a direction parallel to the axis of rotation. The track maytherefore have a height in a direction parallel to the axis of rotationwhich varies along the extent of the track. The first and second trackportions may be arranged concentrically such that an annular gap isformed between the edge surfaces of the first and second track portions.The edge surfaces of the first and second track portions may thereforerespectively comprise an outer radial face of an inner track portion andan inner radial face of an outer track portion. The stabilising elementmay be located within the annular gap and have a curved shape whenviewed from a direction parallel to the axis of rotation, the curvedshape having a radius substantially corresponding to that of the annulargap. The stabilising element may have a first contact surface whichengages the outer radial surface of the inner track portion and has aradius of curvature substantially corresponding to the radius ofcurvature of the outer radial surface of the inner track portion, and asecond contact surface which engages the inner radial surface of theouter track portion and has a radius of curvature substantiallycorresponding to the radius of curvature of the inner radial surface ofthe outer track portion.

The edge surface may be lubricated such that the contact surface of thestabilising element contacts the edge surface of the track via a layerof lubricant. Where the track comprises first and second track portions,the edge surfaces of both track portions may be lubricated.

The contact surface of the stabilising element may comprise a lubricantpick-up adapted to receive lubricant from the edge surface of the track.Lubricant may therefore be taken up by the stabilising element via thelubricant pick-up and supplied around the stabilising element and aroundthe piston to any parts of the stabilising element or piston whichrequire lubrication. The pick-up may comprise a hole formed in thecontact surface. There may be one or more pick-ups located on one orboth contact surfaces of the stabilising element.

The edge surface may be lubricated by a lubricant delivery device;wherein the lubricant delivery device comprises a body portion includinga lubricant delivery port through which lubricant is supplied to theedge surface; wherein the lubricant delivery device further comprises atleast one follower which couples the lubricant delivery device to thetrack such that, as the track moves relative to the cylinder, thelubricant delivery device moves in reciprocating motion in accordancewith the path of the track thereby maintaining the lubricant supply tothe edge surface. The lubricant delivery device preferably comprisesfirst and second followers running on first and second cam surfaces ofthe track which respectively act to move the lubricant delivery devicein opposing directions. In some embodiments, one or both followers maybe adapted to sweep excess oil away from one or more cam surfaces of thetrack on which the piston mounted followers run, although the lubricantdelivery device followers are not required to have any lubricantclearing function. The lubricant delivery device followers preferablyhave the same spacing and radii as the first and second followersmounted to the piston. The lubricant delivery device followers arepreferably integrally formed with the body portion of the lubricationdevice and have sliding bearing surfaces which run on the cam surfacesof the track, but alternatively may comprise bearings or rollers.Alternatively the edge surface(s) may be lubricated in by alternativemeans, for example lubricant jets.

An internal combustion engine may comprise at least one pistonarrangement according to the first aspect of the invention.Alternatively the piston arrangement may be used in another application,for example in a pump. The internal combustion engine may comprisemultiple piston arrangements running from a common track and/or multipletracks driving separate piston arrangements.

A second aspect of the invention provides a piston arrangement includinga piston moveable within a cylinder and a track having a cam surfacewhich is adapted to move relative to the cylinder; wherein the piston iscoupled to the track by a bearing which runs on the cam surface; whereinthe bearing has a roller and a curved bearing surface on which theroller is rotatably mounted such that the roller is held captive betweenthe cam surface and the bearing surface; wherein the bearing surfaceengages the roller via a contact patch which extends only a portion ofthe distance around the circumference of the roller. A pistonarrangement according to the second aspect of the invention may includeat least some of the features described in relation to the first aspectof the invention.

The bearing transfers loads between the piston and the track via theroller. The roller is adapted to rotate on the curved bearing surfaceand roll along the track, although the roller may also experience sliprelative to the track. Because the bearing does not have a full 360degree contact patch between bearing surface and the roller, the bearingadvantageously reduces or prevents bearing whirl.

The curved bearing surface may engage an inner radial surface of theroller.

The curved bearing surface may be provided on a shoe which extends intoa hollow centre of the roller. The shoe may be integrally formed withthe piston. Alternatively the shoe may be otherwise attached to thepiston, for example by fasteners or by a welded joint. There may be oneor more intervening components via which the shoe is connected to thepiston head. The roller may be held captive between the shoe and therunning surface such that it can rotate on the shoe but cannot leave theshoe unless the shoe and the track are moved apart from each other. Theshoe may comprise a single one-piece shoe, or alternatively the shoe maycomprise multiple discrete shoe sections.

The shoe may comprise a lubricant outlet and/or a lubricant pick-up forsupplying a lubricant to the inner radial surface of the roller and/orremoving lubricant from the inner radial surface of the roller. The shoemay therefore lubricate the interface between the curved bearing surfaceand the roller, and the lubricant may be continuously replaced, forexample to control the temperature of the lubricant on the inner radialsurface of the roller. Preferably the lubricant outlet and/or lubricantpick-up are provided on the curved bearing surface.

The roller may have a radially inwardly extending flange located to theside of the shoe. The flange may comprise first and second radiallyinwardly extending flanges on alternate sides of the shoe. The flange(s)may help to retain the roller on the shoe.

The contact patch between the curved bearing surface and the innerradial surface of the roller may extends through an angle of less than180 degrees. Preferably the contact patch extends through an angle ofless than 160 degrees or less than 140 degrees or less than 120 degreesor less than 100 degrees or less than 90 degrees. Alternatively thecontact patch may extend through an angle greater than 180 degrees, forexample 200 degrees or more. Particularly where the roller comprises oneor more radially inwardly extending flanges, a contact patch whichextends through less than 180 degrees may improve ease of installationof the roller on the shoe.

The contact patch between the curved bearing surface and the innerradial surface of the roller may be offset to one side of a planepassing through the central axis of rotation of the roller and alignedwith the direction of movement of the piston within the cylinder. Forexample, at least 55% or at least 60% or at least 70% or at least 80% ofthe contact patch by area may be located to one side of the plane. Acentral point of the contact patch may be offset to one side of theplane, for example by at least 5 degrees or at least 10 degrees or atleast 20 degrees or at least 30 degrees around the central axis ofrotation of the roller. The shoe may be offset to one side of the plane.The contact patch and the shoe may be aligned with or offset towards adirection of peak loading. Offsetting the shoe towards one side of theroller may help to withstand peak loads which are transmitted betweenthe track and the piston via the roller at an angle which is not alignedwith the direction of movement of the component.

The curved bearing surface may engage an outer radial surface of theroller. Because the bearing surface does not extend around the entirecircumference of the roller, a portion of the outer surface of theroller remains exposed, that is the portion of the circumference of theroller around which the curved bearing surface does not extend.Therefore the roller can run on the cam surface while also engaging thecurved bearing surface at a location directly above its interface withthe cam surface. Load transfer between the track and the piston via theroller is therefore improved.

It should be noted that although the curved bearing surface engages theroller via a contact patch which extends only a portion of the distancearound the circumference of the roller, thereby allowing an exposedportion of the roller to engage the cam surface, the roller may alsoengage a further bearing surface which extends around a greater portionof the circumference, for example up to 360 degrees. In this case thefurther bearing surface is located to the side of the roller at alocation where it is not required to leave a portion of the outer radialsurface of the roller exposed so that it can engage the track. Thefurther bearing surface may be continuous with the bearing surfacereferred to above. For example, if the roller extends through the pistonto engage cam surfaces on alternate sides of the piston (as described inmore detail below), the roller may engage cam surfaces on alternatesides of the piston via contact patches which leave a portion of theradial outer surface exposed and also via an intermediate bearingsurface which may extend around the entire circumference of the roller.

The piston may have a head with a working face; wherein the curvedbearing surface is provided on a reverse side of the piston head. Thepiston head may be at least partially hollow or alternatively solid.Alternatively the curved bearing surface engaging the outer radialsurface of the roller may be provided at another location on the pistonor may be provided on a separate component which is attached to thepiston, for example by fasteners or by a welded joint. There may be oneor more intervening components via which the curved bearing surface isconnected to the piston head. The roller may be held captive between thecurved bearing surface and the running surface such that it can rotateon the curved bearing surface but cannot leave the curved bearingsurface unless the curved bearing surface and the track are moved apartfrom each other.

The curved bearing surface engaging the outer radial surface of theroller may comprise a lubricant outlet and/or a lubricant pick-up.

The roller may have an axial end face; wherein a retaining component isremovably attached to the piston, the retaining component being locatedat the axial end face of the roller to prevent the roller from movingwith respect to the piston in a direction aligned with its rotationalaxis beyond the retaining component. The retaining component may, forexample, be attached to a piston head by one or more removablefasteners.

The contact patch between the curved bearing surface and the outerradial surface of the roller may extend through an angle of between 120degrees and 330 degrees. Preferably the contact patch extends through anangle of more than 150 degrees or more than 180 degrees or more than 210degrees or more than 240 degrees. Preferably the contact patch extendsthrough an angle of less than 310 or less than 290 degrees. The contactpatch may for example extend through an angle of approximately 270degrees. At least a portion of the contact patch may, however, extendthrough a smaller angle, for example 90 degrees.

The contact patch between the curved bearing surface and the outerradial surface of the roller may be offset to one side of a planepassing through the central axis of rotation of the roller and alignedwith the direction of movement of the piston within the cylinder. Acentral point of the contact patch may be offset to one side of theplane. The contact patch may be offset to one side of the plane suchthat an exposed portion of the outer surface of the roller which is notengaged by the bearing surface (that is the portion of the circumferenceof the roller around which the curved bearing surface does not extend)is also offset to one side of the plane. By offsetting the exposedportion of the roller to one side, the bearing can allow the geometry ofthe track and the location of the cylinder to be optimised whileavoiding fouling of the track on piston head (or other component onwhich the curved bearing surface is provide). Alternatively or inaddition the bearing surface may be aligned with or offset towards adirection of maximum loading.

The cam surface may be shaped such that, as the track moves relative tothe cylinder, the piston head moves in reciprocating motion within thecylinder in accordance with the path of the cam surface.

At least a portion of the cam surface may be provided with a coating orsurface treatment. The coating or surface treatment may increasehardness and/or reduce friction. Alternatively or in addition at least aportion of the cam surface may be provided on a separate component to amain body of the track, the separate component providing a harder and/orlower friction surface than the main body of the track.

The track may form a continuous loop which rotates relative to thecylinder about an axis of rotation, and the cam surface may form acontinuous surface extending around the loop. Alternatively there may bea break in the cam surface, for example a small discontinuity. The camsurface may be fixed with respect to the track, or alternatively the camsurface may include a fixed portion which is fixed with respect to thetrack and a moving portion which forms part of the cam surface and whichis moveable with respect to the track, for example one or more rollersmounted to the track. The roller may be adapted to rotate about an axisof rotation, and may rotate freely, be partially constrained and/or bedriven.

The cylinder may remain fixed while the track moves relative to thecylinder. Alternatively the cylinder may move relative to a fixed track.

The roller may comprise a protrusion extending beyond its outer radialsurface around its circumference which engages the track to prevent theroller from moving relative to the piston in a direction aligned withthe rotational axis of the roller and/or the track may comprise aprotrusion extending beyond the cam surface around its circumferencewhich engages the roller to prevent the roller from moving relative tothe piston in a direction aligned with the rotational axis of theroller. Where the roller comprises a protrusion, the track may comprisea corresponding recess or chamfer which engages the protrusion. Wherethe track comprises a protrusion the roller may comprise a correspondingrecess or chamfer which engages the protrusion. One or more protrusionsprovided on the roller and/or the track may be located at one or bothaxial ends of the roller and/or at one or more intermediate positionsbetween the axial ends of the roller.

The contact patch may be part cylindrical. Alternatively the contactpatch may additionally have curvature in a second direction aligned withthe axis of rotation of the roller.

The track may further comprise a second cam surface facing in theopposite direction to the first cam surface; wherein the pistonarrangement further comprises a second bearing having a roller and acurved bearing surface on which the roller is rotatably mounted suchthat the roller is held captive between the second cam surface and thebearing surface; wherein the bearing surface engages the roller via acontact patch which extends only a portion of the distance around thecircumference of the roller. The second bearing may have some or all ofthe features described above in relation to the first bearing. Theroller of the first bearing may be adapted to move the piston in a firstdirection in accordance with the path of the first cam surface and theroller of the second bearing may be adapted to move the piston in anopposing second direction in accordance with the path of the second camsurface. The component may therefore be constrained in both directionsand move in reciprocating motion in accordance with the path of thetrack. The opposing first and second running surfaces may oppose eachother by facing outwardly away from each other or alternatively byfacing inwardly towards each other.

The track may comprise first and second track portions located onopposite sides of the piston, each track portion providing a respectivecam surface. A common roller may engage the cam surfaces of both thefirst and second track portions. The common roller may extend throughthe piston. Alternatively, two separate rollers may be provided onalternate sides of the piston, each roller engaging a respective one ofthe first and second track portions. The first and second track portionsmay each comprise a first cam surface and a second cam surface facing inthe opposite direction with first and second bearings operating on eachof the respective first and second cam surfaces.

The track may be a radial track with the cam surface arranged as innerradial or outer radial surface with respect to an axis of rotation ofthe track relative to the cylinder.

The cylinder may have a central axis, wherein the cylinder axis does notpass through the axis of rotation of the track relative to the cylinder.In this case the component does not follow purely radial motion towardsand away from the centre of the track but instead moves at an acuteangle to the radius line. The axis of rotation of the track relative tothe cylinder may be offset from the cylinder axis by a distance equal toat least 10% or at least 25% or at least 50% or at least 75% or at least100% or more than 100% of the piston stroke length. The axis of rotationof the track relative to the cylinder may be offset from the cylinderaxis by an angle of at least 2 degrees or at least 5 degrees or at least10 degrees or at least 15 degrees or up to 20 degrees when taken fromthe rotational axis of the roller. By arranging the cylinder axis at anangle to the axis of rotation of the track the geometry of the pistonarrangement and track may be optimised for efficient transfer of powerfrom the piston to the track and/or vice-versa. Arranging the piston atan angle may also allow the gradient of the track to be optimised.

The track may be an annular track with the cam surface arranged facingin a direction parallel to an axis of rotation of the track relative tothe cylinder. Where such a track comprises first and second trackportions on alternate sides of the component, the first and second trackportions are preferably arranged concentrically such that an annular gapis formed between the edge surfaces of the first and second trackportions. The piston may have a stabilising portion extending betweenthe first and second portions as described in relation to the firstaspect of the invention.

The cylinder may have a central axis, wherein the cylinder axis is notparallel to the axis of rotation of the track relative to the cylinder.The cylinder axis may be offset from the axis of rotation of the trackrelative to the cylinder by at least 2 degrees or at least 5 degrees orat least 10 degrees or at least 15 degrees or up to 20 degrees. Byarranging the cylinder axis at an angle to the axis of rotation of thetrack the geometry of the piston arrangement and track may be optimisedfor efficient transfer of power from the piston to the track and/orvice-versa. Arranging the piston at an angle may also allow the gradientof the track to be optimised.

The roller may comprise a tapered shape such that its diameter at aradially inner-most edge of the cam surface is smaller than its diameterat a radially outer-most edge of the cam surface. The taper accounts forthe difference in path length between the inner-most part of the trackand the outer-most part of the track, thereby allowing the roller toroll more smoothly over the track without skidding or slipping.

An internal combustion engine may comprising a piston operated by abearing arrangement according to the second aspect of the invention.Alternatively the piston arrangement of the second aspect may be used inany other application, for example in a pump.

A third aspect of the invention provides a piston arrangement includinga piston moveable within a cylinder and a track having a cam surfacewhich is adapted to move relative to the cylinder; wherein the piston iscoupled to the track by a bearing which runs on the cam surface; whereinthe bearing comprises a roller and a curved bearing surface; wherein theroller has a radial outer surface which runs on the cam surface; whereinthe curved bearing surface engages an outer redial surface of the rollersuch that the roller is held captive between the bearing surface and thecam surface; wherein the bearing surface engages the roller via acontact patch which extends only a portion of the distance around thecircumference of the roller such that a portion of the radial outersurface of the roller is exposed so that it can run on the cam surface.

A fourth aspect of the invention provides an internal combustion enginecomprising a piston arrangement including a piston coupled to a track;wherein the track is coupled to a shaft and has a cam surface, and thepiston has a follower which runs on the cam surface of the track tocontrol motion of the piston; the engine further comprising a casingincluding first and second casing elements, wherein the first casingelement meets the second casing element at an interface and the firstpiston moves in reciprocating motion within a bore which passes throughthe interface between the first and second casing elements.

The bore within which the piston moves may be provided by a separatecylinder which is attached to the first and second casing elements.Alternatively the bore may be formed directly in the first and secondcasing elements. A bore formed in the first and second casing elementsmay additionally be provided with a cylinder liner. More than one pistonmay be coupled to the track. For example two opposing pistons may becoupled to the track on opposite sides of the shaft, which each pistonmoving within its own respective bores which passes through theinterface between the first and second casing elements. The track ispreferably a radial track as described above in relation to thepreceding aspects of the invention. The track preferably has a centralhole which receives the shaft and is mounted on the shaft by feeding theshaft through the central hole. Relative rotation between the track andthe shaft is preferably achieved by a splined or keyed interface betweenthe tracks and the shaft.

A power transfer mechanism including a piston with a follower coupled toan output shaft by a track with a cam surface is larger and may be moredifficult to assemble than a conventional power transfer mechanism usinga crankshaft and con rod. By adapting the engine casing to be assembledaround the piston and the track after the piston has been coupled to thetrack the engine is easier to design and assemble. By arranging thecasing with a split line between adjacent casing elements at thelocation of the bore, the first and second casing elements may bereadily assembled around the piston and track after the piston has beencoupled to the track and the track has been coupled to the shaft.

The track may have first and second cam surfaces facing in oppositedirections and the piston may have first and second followers whichrespectively run on the first and second cam surfaces of its respectivetrack; wherein the piston is coupled to its respective track by engagingthe first cam surface with the first follower and engaging the secondcam surface with the second follower. The first follower and first camsurface may be adapted to move the piston in a first direction and thesecond follower and second cam surface may be adapted to move the pistonin a second direction opposing the first direction. The track may beheld captive between the first and second followers, or alternativelythe first and second followers may be held captive between the first andsecond cam surfaces.

The track may comprise first and second track portions, each trackportion having a cam surface; wherein the piston is coupled to the trackvia the cam surfaces of the first and second track portions. The camsurfaces of the first and second track portions may each be engaged by acommon follower or alternatively two separate followers may be provided,each follower engaging the cam surface of a respective one of the trackportions. The piston may include a stabilising element extending betweenthe first and second track portions. Each track portion may comprisefirst and second cam surfaces facing in opposite directions, with afollower running on each of the first and second cam surfaces. The trackportions may be separated from each other by a spacer mounted on theshaft, for example an annular sleeve type spacer.

A plate-like locating element may be provided between the first andsecond track portions, the locating element acting to stabilise thepiston and prevent movement of the piston between the first and secondtrack portions.

The interface between the first and second casing elements may besubstantially planar. The interface may lie in a plane which issubstantially perpendicular to the axis of the shaft. Alternatively theinterface may follow any other contour, may not lie in a planeperpendicular to the shaft, and/or may be spliced or include otheroverlapping features.

The internal combustion engine may further comprise a second pistoncoupled to a second track; wherein the second track is coupled to theshaft and has a cam surface, and the second piston has a follower whichruns on the cam surface of the second track to control motion of thesecond piston; the casing further including a third casing element,wherein the second casing element meets the third casing element at aninterface and the second piston moves in reciprocating motion within abore which passes through the interface between the second and thirdcasing elements.

Where an engine comprises multiple tracks operating multiple pistons, aplurality of casing elements are provided along the length of the shaftsuch that each piston moves in reciprocating motion within a boreextending through an interface between two adjacent casing elements. Theengine may generally include any number of tracks spaced apart along theshaft, each track being coupled to one or more pistons. Two casingelements are brought together around the (or each) piston coupled toeach track. Therefore for each additional track the engine includes anadditional casing element such that a casing element is located betweeneach pair of adjacent tracks. For example, an engine having only onetrack will have a pair of end casing elements which meet at an interfacearound the (or each) piston coupled to the single track. Each of the endcasing elements will partially enclose the bore within which the (oreach) piston coupled to the single track moves. An engine having onlytwo tracks will have a pair of end casing elements and an intermediatecasing element located between the pair of tracks, the intermediatecasing element meeting one of the end casing elements at an interfacethrough which the (or each) piston coupled to one of the tracks passesand meeting the other one of the end casing elements at an interfacethrough which the (or each) piston coupled to the other one of thetracks passes. An engine having only three tracks, the casing willinclude a pair of end casing elements, an intermediate casing elementlocated between the first and second tracks and another casing elementlocated between the second and third tracks.

A common design of casing elements may be used in the assembly ofvarious different engines having different numbers of track/pistonarrangements. The number of different components required for producingmultiple different designs of engine may therefore be reduced.

Each casing element may at least substantially surround the shaft. A gapor aperture may be provided in at least one of the casing elements whichopens into a sump. Alternatively at least one of the casing elements maynot extend around the shaft. An end casing element may be open at itsinwardly facing end to interface with the adjacent casing element andclosed at its outwardly facing end to seal the end of the engine casing.An intermediate casing element (located between two adjacent trackswhere the engine comprises multiple tracks) is generally open at bothends to interface with adjacent casing elements on both sides. Thecasing elements may be cast and/or machined.

The casing elements may be attached together by a plurality of fastenerspassing through each of the casing elements. The fasteners may be, forexample, bolts or threaded bars. The casing elements may be attachedtogether by a single set of fasteners passing through all of the casingelements, or alternatively each casing element may be individuallyattached to the adjacent casing element(s). One or more dowel pins mayadditionally pass through at least two adjacent casing elements.

A fifth aspect of the invention provides a method of assembling aninternal combustion engine comprising a piston arrangement including apiston coupled to a track; wherein the track is coupled to a shaft andhas a cam surface, and the piston has a follower which runs on the camsurface of the track to control motion of the piston; the engine furthercomprising a casing including first and second casing elements, whereinthe first casing element meets the second casing element at an interfaceand the first piston moves in reciprocating motion within a bore whichpasses through the interface between the first and second casingelements; the method including the steps of:

a) coupling the first piston arrangement to the shaft by coupling thefirst track to the shaft and coupling the first piston to the firsttrack;

b) bringing the first and second casing elements together around thefirst piston after the first piston arrangement has been coupled to theshaft; and

c) coupling the first and second casing elements together using one ormore fasteners.

The first and second casing elements may both be moved into positionwith respect to the shaft at the same time after the first pistonarrangement has been coupled to the track or alternatively at differenttimes after the first piston arrangement has been coupled to the track.Alternatively either one of the casing elements may be moved intoposition before the first piston arrangement has been coupled to theshaft and the other casing element may be moved into position (such thatthe first and second casing elements are brought together) after thefirst piston arrangement has been coupled to the shaft.

As stated above, a power transfer mechanism including a piston with afollower coupled to an output shaft by a track with a cam surface islarger and may be more difficult to assemble than a conventional powertransfer mechanism using a crankshaft and con rod. By adapting theengine casing to be assembled around the piston and the track after thepiston has been coupled to the track the engine is easier to design andassemble. In particular the technician assembling the engine has enoughspace to couple the first piston to the first track and couple the firsttrack to the shaft because he has full access from at least one side ofthe piston arrangement.

The internal combustion engine may further comprise a second pistoncoupled to a second track; wherein the second track is coupled to theshaft and has a cam surface, and the second piston has a follower whichruns on the cam surface of the second track to control motion of thesecond piston; the casing further including a third casing element,wherein the second casing element meets the third casing element at aninterface and the second piston moves in reciprocating motion within abore which passes through the interface between the second and thirdcasing elements. The method may further include the steps of:

d) coupling the second piston arrangement to the shaft by coupling thesecond track to the shaft and coupling the second piston to the secondtrack;

e) bringing the second and third casing elements together around thesecond piston after the second piston arrangement has been coupled tothe shaft; and

f) coupling the first and second casing elements together and couplingthe second and third casing elements together using one or morefasteners,

wherein the second casing element is positioned between the first andsecond pistons before the first and second piston arrangements have bothbeen coupled to the shaft.

The second casing element (which is an intermediate casing elementlocated between two adjacent piston arrangements associated with twoadjacent separate tracks) is brought into position with respect to theshaft before both of the first and second piston arrangements have beencoupled to the track. In this way it is possible to move the secondcasing element into position without fouling on the tracks of thepistons. The second casing element may be brought into position eitherbefore either of the first and second piston arrangements has beencoupled to the shaft or alternatively after one of the pistonarrangements has been coupled to the shaft but before the other pistonarrangement has been coupled to the shaft.

The first, second and third casing elements may all be coupled togetherat the same time, for example using a common set of fasteners.Alternatively the first and second casing elements may be coupledtogether using a first set of fasteners and the second and third casingelements may be coupled together separately using a second set offasteners.

The method of the fifth aspect of the invention may be used to assemblean engine having any number of tracks spaced apart along a shaft, eachtrack forming part of a piston arrangement. In this case eachintermediate casing element (located between two adjacent tracks) isbrought into position before the piston arrangements on both sides havebeen coupled to the shaft, although generally after the pistonarrangement on one side has been coupled to the shaft. Each end casingelement is brought into position after the piston arrangement arrangedat that far end of the shaft has been brought into position.

Where the engine includes one or more locating elements positionedbetween two adjacent track portions, the method may further comprise thestep of positioning the locating element between the first and secondtrack portions before attaching the plurality of casing elementstogether to form a casing around the piston and the track. The internalcombustion engine of the fourth aspect may comprise a piston arrangementaccording to any of the first, second and/or third aspects of theinvention, and the method of the fifth aspect of the invention may beused to assemble an internal combustion engine comprising a pistonarrangement according to any of the first, second and/or third aspectsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows an engine;

FIG. 2 shows the engine with various components omitted;

FIGS. 3 and 4 show end and top views of the engine with the casingomitted;

FIGS. 5a, 5b, 6a and 6b show the power transfer mechanism for one of thepistons in the engine with certain components omitted;

FIGS. 7a to 7e show a piston from the engine;

FIG. 8 shows a cross section through the power transfer mechanism forone of the pistons in the engine;

FIGS. 9a and 9b shows two of the pistons from the engine and locatingelements used to stabilise the pistons and an oil delivery mechanism;

FIGS. 10a to 10d show various alternative bearing arrangements;

FIG. 11 shows an alternative track which may be used in the engine;

FIGS. 12, 13 a, 13 b and 14 show an alternative track lay-out;

FIGS. 15a and 15b show an alternative bearing arrangement; and

FIGS. 16a to 16j show the engine at various stages during assembly.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1 shown as internal combustion engine 1 comprising a block assembly2 and two head assemblies 3 a, 3 b and an exhaust system. FIG. 2 showsthe engine 1 with the head assemblies 3 a, 3 b and exhaust systemomitted. The head assemblies and exhaust system will not be discussed indetail because they do not form part of the invention. The blockassembly 2 comprises three casing members 4 a, 4 b, 4 c, in which fourpiston assemblies 5 a, 5 b, 5 c, 5 d are mounted. FIGS. 3 and 4respectively show end and top views of the engine 1 with the casingmembers 4 a, 4 b, 4 c, removed so that the piston assemblies 5 a, 5 b, 5c, 5 d are visible. FIGS. 5a, 5b, 6a and 6b show the first pistonassembly 5 a with the second third and fourth piston assemblies 5 b, 5c, 5 d omitted for clarity. The structure and functioning of the firstpiston assembly will be described in detail, although it will beappreciated that the second, third and fourth piston assemblies arestructurally and functionally similar to the first piston assembly. Thefirst piston assembly 5 a comprises a piston 80 with a piston head 7(shown in FIGS. 7a to 7e ) moveable within a cylinder sleeve 8 (visiblein FIG. 2). The piston head 7 is coupled to an output shaft 9 having anaxis of rotation 9 a and an output flange 10 via a track comprisingfirst and second track portions 11 a, 11 b. Each track portion 11 a, 11b is mounted on and rotationally fixed with respect to the output shaft9 via a splined connection (not visible). The track portions 11 a, 11 brotate with the output shaft 9 about the axis of rotation 9 a relativeto the piston head 7 and the cylinder sleeve 8.

Each track portion 11 a, 11 b has a radially outer cam surface 12 a, 12b extending around its circumference. Each track portion also has aradially inner cam surface 13 a, 13 b located on an underside of anoverhanging portion, the inner cam surfaces facing away from the outercam surfaces, i.e. in the opposite direction. A planar edge surface 14a, 14 b extends between the outer and inner cam surfaces of each trackportion.

FIGS. 7a, 7b, 7c and 7d show side, front, rear and perspective views ofthe piston of the first piston assembly 5 a. The piston includes thepiston head 7, a stabilising element or blade element 15 connected toand extending below the piston head, and a cap element 16 connected toand extending below the stabilising element. The stabilising element 15and cap element 16 are connected to the piston head 7 by bolts 17. Thepiston further includes first and second cylindrical rollers 18, 20rotatably mounted within the piston.

The first roller 18 has a cylindrical outer radial surface which bearsagainst a pair of part-cylindrical contact patches 19 a formed on theunderside of the piston head 7 (that is the side opposing the workingface) on each side of the piston (that is on opposite sides of thestabilising element 15). The part-cylindrical contact patches 19 a eachextend only a portion of the distance around the circumference of theroller 18 so they do not fully enclose the roller but instead leaveportions 18 a of the radial outer surface of the roller exposedunderneath the part-cylindrical contact patches 19 a (as shown in FIGS.7a to 7d ). The exposed portions 18 a run on the outer cam surfaces 12a, 12 b of the track 11 a, 11 b and transmit load between the piston andthe track. Since the part-cylindrical bearing surfaces 19 a are locateddirectly above the contact patches between the roller 18 and the outercam surfaces 12 a, 12 b of the track the roller can efficiently transmitload directly between the piston and the track.

The part-cylindrical contact patches 19 a extend through an angle α1 ofapproximately 90 degrees about the axis of rotation of the roller 18 attheir narrowest outer-most portions (that is the ends furthest from themain body of the stabilising element 15) and through an angle ofapproximately 270 degrees at their widest, inner-most portions. Theroller 18 also engages the piston via an additional full-cylindricalcontact patch 19 b located between the part-cylindrical contact patches19 a, as shown in FIG. 7e . However, the full-cylindrical contact patch19 b is located between the cam surfaces 12 a, 12 b at a location wherethe roller does not engage the track and so no portion of the outerradial surface of the roller is required to be left exposed at thislocation.

As can be seen in FIG. 7a , the part-cylindrical contact patches 19 aare not located symmetrically with respect to the central axis 7 a ofthe piston head 7 but instead each of the contact patches is offset toone side of the piston axis when the piston is viewed from the directionof the rotational axis of the roller 18. This offset allows the exposedportions 18 a of the roller to be offset to one side of the piston toallow clearance for the track 11 a, 11 b which would otherwise foul onthe piston. This is especially important in the embodiment shown in FIG.3 where the piston axis is not arranged radially but is insteadangularly offset from the axis of rotation of the track, as can be seenin FIGS. 3 and 5 a and as will be discussed in more detail below. Thefirst roller 18 is prevented from moving axially by keeper plates 22(shown in FIG. 7a ) which are bolted to the piston head at each axialend of the roller.

The second roller 20 has a cylindrical outer radial surface which bearsagainst a pair of part-cylindrical contact patches 21 a formed on theupper side of the cap element 16 on each side of the piston. Thepart-cylindrical contact patches 21 a each extend only a portion of thedistance around the circumference of the roller 20 (through an angle α2)so they do not fully enclose the roller but instead leave portions 20 aof the radial outer surface exposed above the part-cylindrical contactpatches 21 a. The exposed portions 20 a run on the inner cam surfaces 12a, 12 b to transmit load between the piston and the track 11 a, 11 b. Ascan be seen in FIG. 7a , the part-cylindrical contact patches 21 a areoffset in a similar manner to the part-cylindrical contact patches 19 awhich engage the first roller 18. The second roller 20 also has asmaller diameter and a lighter construction than the first rollerbecause the peak loads transmitted between the piston and the track bythe second roller are lower than the peak loads transmitted between thepiston and the track by the first roller.

The stabilising element 15 comprises first and second contact surfaces23 a, 23 b (shown in FIG. 7d ) which respectively engage the edgesurfaces 14 a, 14 b of the track 11 a, 11 b via an oil film to stabilisethe piston relative to the track. The contact surfaces 23 a, 23 b aresubstantially planar to correspond to the planar edge surfaces of thetrack. However, the stabilising element comprises a taper in itsthickness t across its extent in a direction parallel to the directionof relative motion between the stabilising element and the track(indicated by an arrow in FIG. 8) such that the clearance distancebetween the contact surfaces 23 a, 23 b of the stabilising element andthe edge surfaces 14 a, 14 b of the track 11 a, 11 b decreases from thefront to the rear of the stabilising element (the front being defined asthe end which faces towards the oncoming track, i.e. the edge towardsthe top of the page in FIG. 8). The stabilising element may, forexample, have a taper angle α3 of approximately 0.03 degrees. Thetapered shape of the stabilising element 15 causes the stabilisingelement to centre itself under the action of hydrostatic pressure as thetrack moves relative to the piston, thereby stabilising the piston. Across section of the stabilising element and track taken in a planeperpendicular to the axis of movement of the piston is shown in FIG. 8with the taper exaggerated for illustrative purposes.

The stabilising element 15 comprises first and second end surfaces 30,31 between which the contact surfaces 23 a, 23 b extend. The first andsecond end surfaces 30, 31 engage and slide against first and secondlocating surfaces 32 a, 33 a provided on first and second locatingelements 32, 33, as shown in FIG. 9a . (The locating elements are notshown in FIGS. 3 to 6 b to improve clarity.) The locating elements areplate-like elements which are positioned between the first and secondtrack portions 11 a, 11 b and bolted to the casing members 4 a, 4 b, 4 cwhen the engine is fully assembled. (The locating elements are omittedfrom FIGS. 3 to 7 so that the piston tracks and pistons are notobstructed). The locating elements 32, 33 prevent the stabilisingelement 15 from moving forwards and backwards or rotating within theslot formed between the track portions. As can be seen in FIG. 9a , thesame pair of locating elements are also used to locate the stabilisingelement of the piston of the second piston assembly 5 b.

The stabilising element 15 comprises an oil pick-up 24 having openingsin its contact surfaces 23 a, 23 b adapted to receive oil from the oilfilm on the edge surfaces 14 a, 14 b of the track 11 a, 11 b. Oil issupplied from the oil pick-up 24 around the piston via internal oilpassages 25 to locations requiring lubrication, for example theinterface between the first roller 18 and the piston head 7 andstabilising element 15 and the interface between the second roller 20and the stabilising element 15 and cap element 16.

Oil is delivered to the edge surfaces 14 a, 14 b by an oil deliverydevice 50 shown in FIGS. 9a and 9b . The oil delivery device 50comprises an elongate main body portion 51 which is slidably received ina recess 60 formed in the locating element 33, the main body 51 having aworking surface facing away from the locating element 33 and a pair ofcam followers 52, 53 extending from the working surface at either end ofthe body portion. The first cam follower 52 has a bearing surface with aradius of curvature substantially equal to the radius of the firstroller 18 and bears against the outer cam surface 12 a of the firsttrack portion 11 a. The second cam follower 53 has a bearing surfacewith a radius of curvature substantially equal to the radius of thesecond roller 20 and bears against the inner cam surface 13 a of thefirst track portion 11 a. The working surface of the oil delivery devicelies adjacent the edge surface 14 a of the first track portion 11 a. Asthe track rotates within the engine, the followers 52, 53 move the oildelivery device within the recess 60 in accordance with the path shapeof the track such that the working surface remains adjacent the edgesurface of the track. A second oil delivery device (not shown) isprovided in a similar slot 62 formed in the other locating element 32.

Oil is delivered under pressure by a pump (not shown) into the recess 60via an internal passage extending through the locating element 33 andhaving an outlet in the recess. (The outlet of the internal passage isobscured by the main body 51 of the oil delivery device 50 but theoutlet opening into a similar recess for receiving a similar oildelivery device in the other locating element 32 is visible in FIG. 9a.) The oil is received and collected in a chamber 54 formed in thereverse face of the main body 51 (shown in FIG. 9b ), then supplied tothe edge surface 14 a of the first track portion 11 a through a hole 55extending through the main body 51 from the chamber 54 to the workingsurface. The chamber is sufficiently long that the outlet discharges oildirectly into the chamber 54 throughout the full range of motion of theoil delivery device as it is moved by the track 11 a. The workingsurface acts to limit the height of the oil film formed on the edgesurface 14 a, and can act as a wiper to remove excess oil. Therefore theoil delivery device 50 allows oil to be continuously supplied in acontrolled manner to the edge surface 14 a of the track portion 11 ashortly upstream of the stabilising element such that the edge surfaceis well lubricated as it moves past the stabilising element. The secondoil delivery device lubricates the edge surface 14 a of the trackportion 11 a shortly upstream of the stabilising element of the pistonassembly 5 b located on the opposite side of the track 11 a, 11 b.Additional oil delivery devices (not shown) are mounted on the oppositefaces of the locating elements 32, 33 to lubricate the edge surface 14 bof the second track portion 11 b.

As can be seen in FIG. 5a , the central axis of the piston does not passthrough the axis of rotation of the track 11 a, 11 b but instead theaxis of rotation of the track is offset from the piston axis by an angleα4. The angle α4 is approximately 15 degrees when taken from therotational axis of the first roller 18 when the piston is in the centreof its stroke. Therefore the piston head 7 does not follow pure radialmotion with respect to the track 11 a, 11 b. By offsetting the pistonfrom the track in this manner the efficiency with which power istransferred from the piston to the track and/or vice versa may bemaximised. The geometry of the track can also be optimised whileavoiding fouling of the piston on the track.

In operation of the engine, the first roller 18 bears against and rollsalong the outer cam surfaces 12 a, 12 b of the track 11 a, 11 b and thesecond roller 20 bears against and rolls along the inner cam surfaces 13a, 13 b of the track 11 a, 11 b. As the track 11 a 11 b rotates withinthe engine 1, the piston head 7 moves within its cylinder sleeve 8 inaccordance with the path shape of the track. During some portions of theoperating cycle the piston is driven by the track, for example duringintake, compression and exhaust strokes. During some portions of theoperating cycle the track is driven by the piston such that work can beextracted from the output shaft 9, for example during power strokes.Since the path shape of the track may be given any desired shape, thepiston is not constrained to following simple harmonic motion as in mostconventional internal combustion engines but can instead have anydesired displacement profile. The displacement profile may, for example,include multiple different local top dead centre positions at differentheights and/or multiple different bottom dead centre positions atdifferent heights. As can be seen from FIG. 3, the track 11 a, 11 b isshaped such that the piston completes 6 strokes for every output shaftrevolution (including a steam cycle in addition to intake, compression,power and exhaust strokes) and experiences different top dead centrepositions and different bottom dead centre positions at differentheights.

As can be seen from FIG. 4, the second piston arrangement 5 b comprisesa piston coupled to the same track 11 a, 11 b as the first pistonarrangement 5 a by a similar bearing arrangement. The pistons of thefirst and second piston arrangements 5 a, 5 b are operated 180 degreesout of phase with each other. The third and fourth piston arrangements 5c, 5 d respectively comprise pistons coupled to a similar trackcomprising two track portions 11 c, 11 d which are non-rotatably mountedto the output shaft 9. The timings of the piston arrangements 5 a, 5 b,5 c, 5 d are set to substantially cancel engine vibration. Inalternative embodiments an engine may comprise any number of tracks (forexample 1, 2, 3 or more) and may have any number of piston arrangements(for example 1, 2, 3 or more) operated by each track.

In the embodiment described above the track comprises first and secondtrack portions 11 a, 11 b each providing an upper cam surface and alower cam surface. The piston has a first roller 18 which engages theupper surfaces of both track portions, a second roller 20 which engagesthe lower surfaces of both track portions, and a stabilising elementextending between the first and second track portions (see FIG. 10a ).However, the skilled person will appreciate that other track/rollerarrangements are possible, such as those shown in FIGS. 10b to 10d . Forexample, the first and second rollers may each be replaced by twoseparate rollers such that first and second rollers 1018 a, 1020 arespectively engage the upper and lower surfaces of a first trackportion 1011 a and third and fourth rollers 1018 b, 1020 b respectivelyengage the upper and lower surfaces of a second track portion 1011 b, astabilising element 1015 extending between the first and second trackportions (see FIG. 10b ). The rollers 1018 a, 1018 b, 1020 a, 1020 b mayeach extend into the body of the piston head and/or into the body of thestabilising element, or alternatively may each be located outside thebody of the piston head and/or into the body of the stabilising elementbut be constrained between a curved bearing surface and the track. In analternative example a first roller 2018 extends between two parallelstabilising portions 2015 a, 2015 b and engages an upper surface of asingle track portion 2011 while a further pair of rollers 2020 a, 2020 bengages a corresponding pair of lower cam surfaces provided on the sametrack portion (FIG. 10c ). In another alternative example only a singletrack portion 3011 may be provided on a single side of a stabilisingelement 3015 with first and second rollers 3018, 3020 respectivelyengaging upper and lower cam surfaces of the track 3011 (see FIG. 10d ).In this case a locating element may engage the side of the stabilisingelement facing away from the track 3011 to stabilise the piston.

In the embodiments described above the track comprises rigid tracksections providing continuous annular cam surfaces. In an alternativeembodiment the track may include at least one moving part which forms apart of the cam surface and which moves with respect to the rest of thetrack. For example, FIG. 11 illustrates a track 100 having a bearing orroller 101 attached thereto, the roller having an outer radial surface102 which forms a part of the cam surface 103 of the track. The roller101 is located at a local maximum of the track which corresponds to atop dead centre position of a piston operated by the track 100, thepiston having a follower which runs along the cam surface 103. As thefollower moves over the outer radial surface 102 of the roller 101, theroller 101 rotates in the direction of movement of the piston withrespect to the track to prevent or reduce slip between the follower andthe cam surface 103. The roller 101 may be able to rotate through 360degrees or may alternatively be constrained to a smaller angle ofrotation. The roller may be passively driven by motion of the followerover the roller or alternatively may be actively driven, for example bya motor.

In the embodiments described above the track is a radial track whichcontrols motion of the piston via inner and outer radial surfaces. In analternative embodiment a piston 207 may be coupled to an annular track211 a, 211 b with cam surfaces 212 a, 212 b, 213 a, 213 b facing in adirection parallel to the axis of rotation 209 a of the track, forexample as shown in FIG. 12. FIG. 12 shows an annular track comprisingfirst and second track portions 211 a, 211 b each having an edge surface214 a, 214 b engaging opposite sides of a stabilising element of thepiston 207. The track portions 211 a, 211 b are arranged concentricallysuch that an annular gap is formed between the edge surfaces 214 a, 214b of the inner and outer track portions, as shown in FIG. 13a taken fromthe direction of the axis of rotation 209 a. The stabilising element 215of the piston 207 has a curved shape when viewed from a directionparallel to the axis of rotation of the track corresponding to the shapeof the annular gap formed between the edge surfaces 214 a, 214 b, asshown in FIG. 13b (in which the radius of curvature and thickness of thestabilising element have been exaggerated for illustrative purposes).

The central axis of the piston may be offset from the axis of rotationof the track (for example if the axis of rotation of the track lies inthe plane of the page in FIG. 12 then the central axis of the piston isangled out of the page) for example to allow the efficiency of powertransfer from the piston to the track and vice-versa to be maximised.

Where the track is annular, the surface of the rollers 218, 220 may eachhave a tapered shape such that their inner-most end 218 a, 220 a withrespect to the axis of rotation of the track has a smaller diameter thanthe outer-most end 218 a, 220 a, as illustrated in FIG. 14. Theouter-most end of the roller therefore has a greater circumference andwill roll over a longer distance for each revolution of the rollercompared to the inner-most end. In this way the tapered rollers mayaccount for the difference in path length between the radiallyinner-most part of the track and the radially outer-most part of the camsurfaces of the track.

In the embodiments described above the stabilising element is attacheddirectly to the piston head and the first roller bears against theunderside of the piston head. In alternative embodiments the stabilisingelement may be connected to the piston head via an intermediatecomponent such as a con-rod, which may be rigidly attached to the pistonhead and to the stabilising element or alternatively pivotally attachedto the piston head and to the stabilising element.

In the embodiments described above the roller is mounted to the pistonon a curved bearing surface which engages the outer radial surface ofthe roller and extends a portion of the distance around its outercircumference. Additionally (or alternatively) in other embodiments aroller 318 a may be mounted to a piston 307 on a curved bearing surfacewhich engages an inner radial surface of the roller, for example asshown in FIG. 15a . In an alternative embodiment shown in FIGS. 15a and15b a first roller 318 a is mounted to the piston 307 via a curvedbearing surface provided on a shoe 319 a which extends outwardly from astabilising element 315 of the piston into the hollow centre of theroller 318 a and engages the radial inner surface of the roller. Thefirst roller 318 a is also mounted on the piston 307 via a curvedbearing surface provided on the underside of the piston head whichengages the radial outer surface of the roller. The roller 318 atransfers load between the piston 307 and a track portion 311 a via bothcurved bearing surfaces. As can be seen from FIG. 15a , the shoe 319 ais offset to one side of the piston 307 so that the centre of thecontact patch between the roller and the shoe is aligned with thedirection of maximum loading. The roller 318 a has radially inwardlyextending flanges located on opposite sides of the shoe 319 a forretaining the roller axially on the shoe, as shown in FIG. 15b . Theshoe may deliver oil to the inner radial surface of the roller from anoil pick-up located on the contact surface of the stabilising element. Asimilar roller arrangement is provided on the opposite side of thestabilising element with roller 318 b mounted on a shoe 319 b andrunning on a track portion 311 b. Similar roller arrangements 320 a/321a and 320 b/321 b are also provided on the opposite side of the track tomove the piston 307 in the opposite direction. In other embodiments therollers may engage the piston via their radial inner surfaces only sothat the roller is mounted on the shoe only and transfers load betweenthe piston and the track only via the shoe.

The engine is assembled by the following steps. The method recited belowdoes not include all steps required to assemble all of the componentsforming part of the engine but has instead been reduced to the majorsteps for assembling the power transfer mechanism and casing forclarity:

a) Press bearing A into a recess formed in casing element 4 b and insertshaft 9 through a hole formed through casing element 4 b (FIGS. 16a and16b ).

b) Feed track portion 11 b over shaft 9 and bring it into engagementwith bearing A (FIG. 16c ).

c) Couple the piston 80 to the track portion 11 b by engaging the outersurface 12 b with the upper roller 18 and the inner surface 13 b withthe lower roller 20 and feed cylinder sleeve 8 over the piston head(FIGS. 16d and 16e ). Similarly couple the opposed piston of the secondpiston arrangement to the track portion 11 b.

d) Attach the locating elements 32, 33 to the casing element 4 b (FIG.16f ), with oil delivery devices located in their respective slots (oneon each side of each stabilising element).

e) Feed a spacer element S (visible adjacent the track portion 11 b inFIG. 6b ) over shaft 9 and then feed track portion 11 a over the shaftand bring it into engagement with the spacer element (FIG. 16g ).

f) Press bearing B (visible below the track portion 12 a in FIG. 4) intocasing element 4 c and then feed casing element 4 c over shaft 9 tobring the bearing B into engagement with the track portion 11 a (FIG.16h ).

g) Perform equivalent steps to couple the piston arrangements 5 c and 5d to shaft 9 and bring casing element 4 a into place (FIG. 16i ).

h) Attach casing elements 4 a, 4 b and 4 c together with fastenersextending through the casing (FIG. 16j ).

The casing element 4 b which lies between the tracks 11 a, 11 b and 1 c,11 d is brought into place around the shaft 9 before both of the tracksand their respective pistons have been coupled to the shaft. The casingelements 4 b and 4 c are brought together after the piston assemblies 5a and 5 b have been coupled to the shaft 9, and the casing elements 4 aand 4 b are brought together after the piston assemblies 5 c and 5 dhave been coupled to the shaft 9.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

The present disclosure may include one or more of the followingconcepts:

-   A. A piston arrangement including a track and a piston moveable    within a cylinder; wherein the track is adapted to rotate relative    to the cylinder about an axis of rotation and has a cam surface and    an edge surface extending away from the cam surface; wherein the    piston is coupled to the track by a follower running on the cam    surface; wherein the cam surface is shaped such that, as the track    moves relative to the cylinder, the piston head moves in    reciprocating motion within the cylinder in accordance with the path    of the cam surface; wherein a stabilising element is connected to    the piston, the stabilising element extending below the piston head    and comprising a contact surface which engages the edge surface of    the track.-   B. A piston arrangement in accordance with paragraph A, wherein the    track rotates about the axis of rotation while the cylinder remains    stationary with respect to the axis of rotation.-   C. A piston arrangement in accordance with paragraphs A or B,    wherein the stabilizing element is rigidly attached to the piston    head by one or more fasteners.-   D. A piston arrangement in accordance with any of the previous    paragraphs, wherein the shape of the contact surface of the    stabilising element substantially corresponds to the shape of the    edge surface of the track.-   E. A piston arrangement in accordance with any of the previous    paragraphs, wherein the stabilising element comprises first and    second end surfaces, the contact surface extending between the first    and second end surfaces, wherein at least one of the first and    second end surfaces slidingly engages a locating element as the    piston moves in reciprocating motion.-   F. A piston arrangement in accordance with any of the previous    paragraphs, wherein the follower comprises a roller mounted to the    piston, the roller being adapted to rotate relative to the piston as    it runs along the track.-   G. A piston arrangement in accordance with any of the previous    paragraphs, wherein the track comprises first and second cam    surfaces facing away from each other in opposite directions, the    edge surface extending between the first and second cam surfaces;    wherein the piston comprises first and second followers respectively    running on the first and second cam surfaces.-   H. A piston arrangement in accordance with any of the previous    paragraphs, wherein the track comprises first and second track    portions located on opposite sides of the stabilising element,    wherein the first track portion comprises an edge surface extending    away from at least one cam surface and the second track portion    comprises an edge surface extending away from at least one cam    surface and facing towards the edge surface of the first track    portion, wherein the stabilising element comprises first and second    contact surfaces which engage the respective edge surfaces of the    first and second track portions.-   I. A piston arrangement in accordance with any of the previous    paragraphs, wherein the stabilising element has a thickness which    tapers across the extent of the stabilising element in a direction    parallel to the direction of relative movement between the piston    and the track.-   J. A piston arrangement in accordance with any of the previous    paragraphs, wherein the track is a radial track with the cam    surface(s) arranged as inner radial and/or outer radial surfaces    with respect to the axis of rotation.-   K. A piston arrangement in accordance with paragraph J when    dependent on paragraph H, wherein the edge surface of the first    track portion is substantially parallel to the edge surface of the    second track portion.-   L. A piston arrangement in accordance with paragraph K, wherein the    stabilising element is substantially straight when viewed from a    direction parallel to the axis of the piston.-   M. A piston arrangement according to any of paragraphs A to I,    wherein the track is an annular track with the cam surface(s)    arranged facing in a direction parallel to the axis of rotation.-   N. A piston arrangement in accordance with paragraph M when    dependent on paragraph H, wherein the first and second track    portions are arranged concentrically such that an annular gap is    formed between the edge surfaces of the first and second track    portions.-   O. A piston arrangement in accordance with paragraph N, wherein the    stabilising element is located within the annular gap and has a    curved shape when viewed from a direction parallel to the axis of    rotation, the curved shape having a radius substantially    corresponding to that of the annular gap.-   P. A piston arrangement in accordance with any of the previous    paragraphs, wherein the edge surface is lubricated such that the    contact surface of the stabilising element contacts the edge surface    of the track via a layer of lubricant.-   Q. A piston arrangement in accordance with paragraph P, wherein the    contact surface of the stabilising element comprises an oil pick-up    adapted to receive oil from the edge surface of the track.-   R. A piston arrangement in accordance with paragraphs P or Q,    wherein the edge surface is lubricated by a lubricant delivery    device; wherein the lubricant delivery device comprises a body    portion including a lubricant delivery port through which lubricant    is supplied to the edge surface; wherein the lubricant delivery    device further comprises at least one follower which couples the    lubricant delivery device to the track such that, as the track moves    relative to the cylinder, the lubricant delivery device moves in    reciprocating motion in accordance with the path of the track    thereby maintaining the lubricant supply to the edge surface.-   S. An internal combustion engine comprising at least one piston    arrangement in accordance with any of the previous paragraphs.-   T. A piston arrangement including a piston moveable within a    cylinder and a track having a cam surface which is adapted to move    relative to the cylinder; wherein the piston is coupled to the track    by a bearing which runs on the cam surface; wherein the bearing has    a roller and a curved bearing surface on which the roller is    rotatably mounted such that the roller is held captive between the    cam surface and the bearing surface; wherein the bearing surface    engages the roller via a contact patch which extends only a portion    of the distance around the circumference of the roller.-   U. A piston arrangement in accordance with paragraph T, wherein the    curved bearing surfaces engages an inner radial surface of the    roller.-   V. A piston arrangement in accordance with paragraph U, wherein the    curved bearing surface is provided on a shoe which extends into a    hollow centre of the roller.-   W. A piston arrangement in accordance with paragraph V, wherein the    shoe comprises a lubricant outlet and/or a lubricant pick-up for    supplying a lubricant to the inner radial surface of the roller    and/or removing lubricant from the inner radial surface of the    roller.-   X. A piston arrangement in accordance with paragraphs V or W,    wherein the roller has a radially inwardly extending flange located    to the side of the shoe.-   Y. A piston arrangement according to any of paragraphs U to X,    wherein the contact patch between the curved bearing surface and the    inner radial surface of the roller extends through an angle of less    than 180 degrees.-   Z. A piston arrangement according to any of paragraphs U to Y,    wherein the contact patch between the curved bearing surface and the    inner radial surface of the roller is offset to one side of a plane    passing through the central axis of rotation of the roller and    aligned with the direction of movement of the piston within the    cylinder.-   AA. A piston arrangement according to any of paragraphs T to Z,    wherein the curved bearing surface engages an outer radial surface    of the roller.-   BB. A piston arrangement in accordance with paragraph AA, wherein    the piston has a head with a working face; wherein the curved    bearing surface is provided on a reverse side of the piston head.-   CC. A piston arrangement in accordance with paragraphs AA or BB,    wherein the curved bearing surface engaging the outer radial surface    of the roller comprises a lubricant outlet and/or a lubricant    pick-up.-   DD. A piston arrangement according to any of paragraphs AA to CC,    wherein the roller has an axial end face; wherein a retaining    component is removably attached to the piston, the retaining    component being located at the axial end face of the roller to    prevent the roller from moving with respect to the piston in a    direction aligned with its rotational axis beyond the retaining    component.-   EE. A piston arrangement according to any of paragraphs AA to DD,    wherein the contact patch between the curved bearing surface and the    outer radial surface of the roller extends through an angle of    between 120 degrees and 330 degrees.-   FF. A piston arrangement according to any of paragraphs AA to EE,    wherein the contact patch between the curved bearing surface and the    outer radial surface of the roller is offset to one side of a plane    passing through the central axis of rotation of the roller and    aligned with the direction of movement of the piston within the    cylinder.-   GG. A piston arrangement according to any of paragraphs T to FF,    wherein the cam surface is shaped such that, as the track moves    relative to the cylinder, the piston head moves in reciprocating    motion within the cylinder in accordance with the path of the cam    surface.-   HH. A piston arrangement according to any of paragraphs T to GG,    wherein at least a portion of the cam surface is provided with a    coating or surface treatment.-   II. A piston arrangement according to any of paragraphs T to HH,    wherein the track forms a continuous loop and rotates relative to    the cylinder about an axis of rotation, and wherein the cam surface    forms a continuous surface extending around the loop.-   JJ. A piston arrangement according to any of paragraphs T to II,    wherein the cylinder remains fixed while the track moves relative to    the cylinder.-   KK. A piston arrangement according to any of paragraphs T to JJ,    wherein the roller comprises a protrusion extending beyond its outer    radial surface around its circumference which engages the track to    prevent the roller from moving relative to the piston in a direction    aligned with the rotational axis of the roller and/or wherein the    track comprises a protrusion extending beyond the cam surface around    its circumference which engages the roller to prevent the roller    from moving relative to the piston in a direction aligned with the    rotational axis of the roller a recess or chamfer or protrusion in    its outer radial surface extending around its circumference; wherein    the track comprises a corresponding protrusion or recess or chamfer    which engages the recess or chamfer or protrusion of the roller to    prevent to roller from moving relative to the piston in a direction    aligned with its rotational axis.-   LL. A piston arrangement according to any of paragraphs T to KK,    wherein the contact patch is part cylindrical.-   MM. A piston arrangement according to any of paragraphs T to LL,    wherein the track further comprises a second cam surface facing in    the opposite direction to the first cam surface; wherein the piston    arrangement further comprises a second bearing having a roller and a    curved bearing surface on which the roller is rotatably mounted such    that the roller is held captive between the second cam surface and    the bearing surface; wherein the bearing surface engages the roller    via a contact patch which extends only a portion of the distance    around the circumference of the roller.-   NN. A piston arrangement according to any of paragraphs T to MM,    wherein the track comprises first and second track portions located    on opposite sides of the piston, each track portion providing a    respective cam surface.-   OO. A piston arrangement according to any of paragraphs T to NN,    wherein the track is a radial track with the cam surface arranged as    inner radial or outer radial surface with respect to an axis of    rotation of the track relative to the cylinder.-   PP. A piston arrangement in accordance with paragraph OO, wherein    the cylinder has a central axis, wherein the cylinder axis does not    pass through the axis of rotation of the track relative to the    cylinder.-   QQ. A piston arrangement according to any of paragraphs T to NN,    wherein the track is an annular track with the cam surface arranged    facing in a direction parallel to an axis of rotation of the track    relative to the cylinder.-   RR. A piston arrangement in accordance with paragraph QQ, wherein    the cylinder has a central axis, wherein the cylinder axis is not    parallel to the axis of rotation of the track relative to the    cylinder.-   SS. A piston arrangement in accordance with paragraphs QQ or RR,    wherein the roller comprises a tapered shape such that its diameter    at a radially inner-most edge of the cam surface is smaller than its    diameter at a radially outer-most edge of the cam surface.-   TT. An internal combustion engine comprising a piston operated by a    bearing arrangement according to any of paragraphs T to SS.-   UU. A piston arrangement including a piston moveable within a    cylinder and a track having a cam surface which is adapted to move    relative to the cylinder; wherein the piston is coupled to the track    by a bearing which runs on the cam surface; wherein the bearing    comprises a roller and a curved bearing surface; wherein the roller    has a radial outer surface which runs on the cam surface; wherein    the curved bearing surface engages an outer redial surface of the    roller such that the roller is held captive between the bearing    surface and the cam surface; wherein the bearing surface engages the    roller via a contact patch which extends only a portion of the    distance around the circumference of the roller such that a portion    of the radial outer surface of the roller is exposed so that it can    run on the cam surface.-   VV. An internal combustion engine comprising a piston arrangement    including a piston coupled to a track; wherein the track is coupled    to a shaft and has a cam surface, and the piston has a follower    which runs on the cam surface of the track to control motion of the    piston; the engine further comprising a casing including first and    second casing elements, wherein the first casing element meets the    second casing element at an interface and the first piston moves in    reciprocating motion within a bore which passes through the    interface between the first and second casing elements.-   WW. An internal combustion engine in accordance with paragraph VV;    wherein the track has first and second cam surfaces facing in    opposite directions and the piston has first and second followers    which respectively run on the first and second cam surfaces of its    respective track; wherein the piston is coupled to its respective    track by engaging the first cam surface with the first follower and    engaging the second cam surface with the second follower.-   XX. An internal combustion engine in accordance with paragraphs VV    or WW; wherein the track comprises first and second track portions,    each track portion having a cam surface; wherein the piston is    coupled to the track via the cam surfaces of the first and second    track portions.-   YY. An internal combustion engine in accordance with paragraph XX;    wherein a plate-like locating element is provided between the first    and second track portions, the locating element acting to stabilise    the piston and prevent movement of the piston between the first and    second track portions.-   ZZ. An internal combustion engine according to any of paragraphs VV    to YY; wherein the interface between the first and second casing    elements is substantially planar.-   AAA. An internal combustion engine according to any of paragraphs VV    to ZZ, further comprising a second piston coupled to a second track;    wherein the second track is coupled to the shaft and has a cam    surface, and the second piston has a follower which runs on the cam    surface of the second track to control motion of the second piston;    the casing further including a third casing element, wherein the    second casing element meets the third casing element at an interface    and the second piston moves in reciprocating motion within a bore    which passes through the interface between the second and third    casing elements.-   BBB. An internal combustion engine according to any of paragraphs VV    to AAA; wherein each casing element surrounds the shaft.-   CCC. An internal combustion engine according to any of paragraphs VV    to BBB; wherein the casing elements are attached together by a    plurality of fasteners passing through each of the casing elements.-   DDD. A method of assembling an internal combustion engine comprising    a piston arrangement including a piston coupled to a track; wherein    the track is coupled to a shaft and has a cam surface, and the    piston has a follower which runs on the cam surface of the track to    control motion of the piston; the engine further comprising a casing    including first and second casing elements, wherein the first casing    element meets the second casing element at an interface and the    first piston moves in reciprocating motion within a bore which    passes through the interface between the first and second casing    elements; the method including the steps of: a) coupling the first    piston arrangement to the shaft by coupling the first track to the    shaft and coupling the first piston to the first track; b) bringing    the first and second casing elements together around the first    piston after the first piston arrangement has been coupled to the    shaft; and c) coupling the first and second casing elements together    using one or more fasteners.-   EEE. A method in accordance with paragraph DDD, wherein the internal    combustion engine further comprises a second piston coupled to a    second track; wherein the second track is coupled to the shaft and    has a cam surface, and the second piston has a follower which runs    on the cam surface of the second track to control motion of the    second piston; the casing further including a third casing element,    wherein the second casing element meets the third casing element at    an interface and the second piston moves in reciprocating motion    within a bore which passes through the interface between the second    and third casing elements; the method including the steps of: d)    coupling the second piston arrangement to the shaft by coupling the    second track to the shaft and coupling the second piston to the    second track; e) bringing the second and third casing elements    together around the second piston after the second piston    arrangement has been coupled to the shaft; and f) coupling the first    and second casing elements together and coupling the second and    third casing elements together using one or more fasteners; wherein    the second casing element is positioned between the first and second    pistons before the first and second piston arrangements have both    been coupled to the shaft.

What is claimed is:
 1. A piston assembly comprising: a track; and apiston moveable within a cylinder, the piston having a piston head and apiston axis; wherein the track is rotatable relative to the cylinderabout an axis of rotation extending perpendicular to the piston axis andthe track has a cam surface and an edge surface extending away from thecam surface; wherein the piston is coupled to the track by a followerrunning on the cam surface; wherein the cam surface is shaped such that,as the track moves relative to the cylinder, the piston head moves inreciprocating motion within the cylinder along the piston axis inaccordance with a path of the cam surface; wherein a stabilizing elementis connected to the piston, the stabilizing element extending below thepiston head and including a contact surface engaging the edge surface ofthe track; wherein the track comprises a first cam surface and a secondcam surface facing away from each other in opposite directions; andwherein the piston comprises first and second followers respectivelyrunning on the first and second cam surfaces.
 2. The piston assemblyaccording to claim 1, wherein the track rotates about the axis ofrotation of the track while the cylinder remains stationary with respectto the axis of rotation of the track.
 3. The piston assembly accordingto claim 1, wherein the follower comprises a roller mounted to thepiston, and wherein the roller is rotatable relative to the piston asthe roller runs along the track.
 4. The piston assembly according toclaim 1, wherein the track is a radial track with the first and secondcam surfaces arranged as an inner radial surface and an outer radialsurfaces with respect to the axis of rotation of the track.
 5. Aninternal combustion engine comprising at least one piston assemblyaccording to claim
 1. 6. The piston assembly according to claim 1,wherein the edge surface extends between the first and second camsurfaces.
 7. The piston assembly according to claim 1, wherein thepiston head moves in reciprocating non-simple harmonic motion within thecylinder along the piston axis in accordance with the path of the camsurface.
 8. The piston assembly according to claim 1, wherein thestabilizing element is rigidly attached to the piston head by one ormore fasteners.
 9. The piston assembly according to claim 1, wherein theshape of the contact surface of the stabilizing element substantiallycorresponds to the shape of the edge surface of the track.
 10. A pistonassembly comprising: a track; and a piston moveable within a cylinder,the piston having a piston head and a piston axis; wherein the track isrotatable relative to the cylinder about an axis of rotation extendingperpendicular to the piston axis and the track has a cam surface and anedge surface extending away from the cam surface; wherein the piston iscoupled to the track by a follower running on the cam surface; whereinthe cam surface is shaped such that, as the track moves relative to thecylinder, the piston head moves in reciprocating motion within thecylinder along the piston axis in accordance with a path of the camsurface; wherein a stabilizing element is connected to the piston, thestabilizing element extending below the piston head and including acontact surface engaging the edge surface of the track; wherein thestabilizing element comprises first and second end surfaces, the contactsurface extending between the first and second end surfaces, and whereinat least one of the first and second end surfaces slidingly engages alocating element as the piston moves in reciprocating motion.
 11. Apiston assembly comprising: a track; and a piston moveable within acylinder, the piston having a piston head and a piston axis; wherein thetrack is rotatable relative to the cylinder about an axis of rotationextending perpendicular to the piston axis and the track has a camsurface and an edge surface extending away from the cam surface; whereinthe piston is coupled to the track by a follower running on the camsurface; wherein the cam surface is shaped such that, as the track movesrelative to the cylinder, the piston head moves in reciprocating motionwithin the cylinder along the piston axis in accordance with a path ofthe cam surface; wherein a stabilizing element is connected to thepiston, the stabilizing element extending below the piston head andincluding a contact surface engaging the edge surface of the track;wherein the track comprises first and second track portions located onopposite sides of the stabilizing element, wherein the first trackportion comprises an edge surface extending away from at least one camsurface and the second track portion comprises an edge surface extendingaway from at least one cam surface and facing towards the edge surfaceof the first track portion, and wherein the stabilizing elementcomprises first and second contact surfaces which engage the respectiveedge surfaces of the final and second track portions.
 12. The pistonassembly according to claim 11, wherein the edge surface of the firsttrack portion is parallel to the edge surface of the second trackportion.
 13. The piston assembly according to claim 12, wherein thestabilizing element is straight when viewed from a direction parallel tothe axis of the piston.
 14. A piston assembly comprising: a track; and apiston moveable within a cylinder, the piston having a piston head and apiston axis; wherein the track is rotatable relative to the cylinderabout an axis of rotation extending perpendicular to the piston axis andthe track has a cam surface and an edge surface extending away from thecam surface; wherein the piston is coupled to the track by a followerrunning on the cam surface; wherein the cam surface is shaped such that,as the track moves relative to the cylinder, the piston head moves inreciprocating motion within the cylinder along the piston axis inaccordance with a path of the cam surface; wherein a stabilizing elementis connected to the piston, the stabilizing element extending below thepiston head and including a contact surface engaging the edge surface ofthe track; and wherein the stabilizing element has a thickness whichtapers across an extent of the stabilizing element in a directionparallel to a direction of relative movement between the piston and thetrack.
 15. The piston assembly according to claim 1, wherein the edgesurface is lubricated such that the contact surface of the stabilizingelement contacts the edge surface of the track via a layer of lubricant.16. A piston assembly comprising: a track; and a piston moveable withina cylinder, the piston having a piston head and a piston axis; whereinthe track is rotatable relative to the cylinder about an axis ofrotation extending perpendicular to the piston axis and the track has acam surface and an edge surface extending away from the cam surface;wherein the piston is coupled to the track by a follower running on thecam surface; wherein the cam surface is shaped such that, as the trackmoves relative to the cylinder, the piston head moves in reciprocatingmotion within the cylinder along the piston axis in accordance with apath of the cam surface; wherein a stabilizing element is connected tothe piston, the stabilizing element extending below the piston head andincluding a contact surface engaging the edge surface of the track;wherein the edge surface is lubricated such that the contact surface ofthe stabilizing element contacts the edge surface of the track via alayer of lubricant; and wherein the contact surface of the stabilizingelement comprises an oil pick-up configured to receive oil from the edgesurface of the track.
 17. A piston assembly comprising: a track; and apiston moveable within a cylinder, the piston having a piston head and apiston axis; wherein the track is rotatable relative to the cylinderabout an axis of rotation extending perpendicular to the piston axis andthe track has a cam surface and an edge surface extending away from thecam surface; wherein the piston is coupled to the track by a followerrunning on the cam surface; wherein the cam surface is shaped such that,as the track moves relative to the cylinder, the piston head moves inreciprocating motion within the cylinder along the piston axis inaccordance with a path of the cam surface; wherein a stabilizing elementis connected to the piston, the stabilizing element extending below thepiston head and including a contact surface engaging the edge surface ofthe track; wherein the edge surface is lubricated by a lubricantdelivery device such that the contact surface of the stabilizing elementcontacts the edge surface of the track via a layer of lubricant; whereinthe lubricant delivery device comprises a body portion including alubricant delivery port through which lubricant is supplied to the edgesurface; and wherein the lubricant delivery device further comprises atleast one lubricant delivery device follower which couples the lubricantdelivery device to the track such that, as the track moves relative tothe cylinder, the lubricant delivery device moves in reciprocatingmotion in accordance with the path of the track thereby maintaining thelubricant supply to the edge surface.